<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div {
        width: 100px;
        height: 100px;
        background-color: #000;
        mask: radial-gradient(at right top, transparent 99px, red 100px)top left;
        /* 锯齿渐变差一个像素就好了 */
        mask-size: 100px 100px;
        mask-repeat: no-repeat;
        margin-bottom: 10px;
    }

    div:nth-child(2) {
        mask: radial-gradient(at right top, transparent 64px, red 65px)top left;
    }

    div:nth-child(3) {
        mask: radial-gradient(at right top, transparent 80px, red 100px)top left;
    }

    div:nth-child(4) {
        mask: radial-gradient(at right top, transparent 15px, red 100px)top left;
    }

</style>

<body>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
</body>

</html>